Since the 1990s, Beghal had been a follower of Abu Qatada, the Muslim cleric described as Osama bin Laden’s spiritual ambassador in Europe.

He was such a high-profile prisoner that he had been placed in isolation, the facility’s former imam, Khalil Merroun, told Reuters.

Yet Beghal was able to influence Coulibaly and Kouachi inside prison and pursue contacts with them — in some cases under the nose of intelligence agencies — after leaving prison, potentially sowing the seeds for the Charlie Hebdo and kosher supermarket massacres. Beghal has denied through his lawyer any involvement in the Paris attacks.

“As I understand it Coulibaly was in a cell above Beghal and able to communicate through so-called ‘yo-yos’ dropped from the window or through a folded messages in the prison yard,” said Hamm. “The ratio of guards to inmates makes it impossible to provide anything resembling total surveillance, even in the U.S. where that ratio is higher.”